Piezoelectric crystal



March 19, 1940. Q MATTlAT 2,194,183

PIEZOELECTRIG CRYSTAL Filed June 26, 1937 INVENTOR OSKAR MA TT/A 7 BY 7k g t v01,

ATTORNEY Patented Mar. 19, 1940 UNITED STATES PIEZOELECTRIC CRYSTAL Oskar Mattiat, Berlin,

Germany, assignor to Allgemeine Elektricitats Gesellschaft, Berlin, Germany, a corporation of Germany Application June 26, 1937, Serial No. 150,527 In Germany June 29, 1936 1 Claim.

This invention is particularly adapted to monitor circuits of radio frequency transmitters and also is used in connection with cable filters, whereas in the past quartz crystals have been 5 customarily used for such circuits. The use of quartz crystals appeared necessary for the reason that for a long time it has been the only readily workable crystal possessing a sufficiently high piezo electric constant.

Now, it has been known in the prior art to out crystals out of Rochelle salt mother crystals and to use the same in microphones; but it seems also advantageous to use Rochelle salt crystals and other piezo electric crystals such as urotropin as oscillatory crystals, since these crystals are artificially cultivable, as it were, so that all such difiiculties may be obviated as have been encountered in the production of crystals of suflicient purity.

According to this invention, rodlets are out out of the mother crystal of piezo electric salts to be used as oscillatory crystals in such a way that their longitudinal directional will coincide with the direction of least elasticity expressed by modulus of elasticity in tension, or Youngs modulus.

The invention shall be explained by reference to the drawing in connection with a Rochelle salt crystal, in which:

Fig. 1 illustrates a Rochelle salt crystal with the conventional crystallographic axis designations a, b, c; and

Fig. 2 is a plan view of a plate cut from the crystal.

Referring now in detail to the drawing:

In the case of Rochelle salt, the least modulus of elasticity occurs in the plane of the a-axis and c-axis at an angle of degrees to both axes. Hence, according to the invention a plate P must be cut out at right angles to the b-axis. The same is again illustrated in Fig, 2. The broken-line curve E indicates the size of the modulus of elasticity in terms of polar coordinates. It can be seen therefrom that the lowest elasticity modulus, as stated above, occurs at an angle of 45 degrees. S is intended to show in what way a crystal rod according to the invention should be cut out of the plate P.

Rodlets according to the invention are especially suited as oscillatory crystals. The rod cut out of a Rochelle salt mother crystal according to the instructions hereinbefore disclosed must be of a length of one cm. to be suited for a wave- 55 length of 2100 meters, whereas a quartz crystal for the same wave-length must be two cm. long. In other words, twice as long. If it were desired to cut a crystal out of a Rochelle salt mother crystal, say, in the direction of the c-axis, crystal sizes would be required which are the same as those of the quartz for the same wave-length. Owing to the reduced size, one obtains crystals which, even in the presence of an inherently lower mechanical strength, will exhibit adequate stability in oscillatory state and will thus not break or crack. Tests and research of which the present invention has been the outcome have demonstrated that a crystal of Rochelle salt which had been cut in accordance with the in structions here disclosed is capable of withstanding a load of 15 volts, which is far above the usual stress and load which lies around 1 volt and below.

Another practical merit residing in crystals cut in accordance with this invention is that oscillations due to thrust which would occur in a plate out, say, in the way of plate P, Fig. 2, will thus be avoided so that pure elongation oscillations are obtained. The consequence is an increased purity of the oscillations. The excitation of the crystals, similarly as in quartz, is produced by the aid of two electrodes placed upon the rod, these electrodes being positioned, as will be noted, parallel to the plane through the drawing, Fig. 2. However, while in the quartz crystals vibrations occur in the sense of the electric force, both the undesirable thrust motions of plate P as well as the desirable, elongational vibrations of the rod 8 occur at right angles to the electrical force.

Crystal rods out according to the invention are but little dependent upon temperature efiect so far as their elasticity modulus and thus their natural frequency is concerned. Under certain circumstances, however, the crystals may be safeguarded from temperature and humidity infiuences by embedding them in coats .of nonhygroscopic substances which are of a nature not to affect the oscillatory properties of the crystal. What has been found serviceable for this purpose are copals.

In order to preclude the chance of producing harmonics and coupling oscillations, the width of the crystal must bear a certain relationship to the length thereof. It has been discovered that suitable oscillatory crystals are obtained if the breadth of the rods is less than one-third their length.

What is claimed is:

A crystal rod cut from piezo electric salts especially Rochelle salt adapted to be used for direction coincides with the direction of the oscillatory crystal Work, with this characteristic lowest Youngs modulus of elasticity and that feature that it is cut out of the mother crystal the Width of said crystal rod is less than oneat right angles to the crystallographic b-axis at third its length.

an angle of forty-five degrees to the crystallo- OSKAR MATTIAT. 5 graphic a-axis and c-axis so that its longitudinal 

